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Sunday, 21 December 2014

An azhdarchid pterosaur takes off in a festively-coloured woodland, because it's Christmas. Prints are available.

I was recently thrilled to have the above image featured on the front cover of The Anatomical Record, its depiction of a freshly launched azhdarchid pterosaur tying in with the first paper of the issue. And yes, the colours are deliberately festive, because the Anatomical Record adopts a Christmas theme for its December issues, hence the deep reds and greens of my image. There's two stories I briefly want to tell about this: one about the paper it accompanies, and the other about the art itself.

The paper

My art accompanies the work of Nick Geist and his team on the respiratory mechanism of large pterodactyloid pterosaurs (Geist et al. 2014). Lung ventilation in pterosaurs is an interesting topic. The torso skeleton of many pterodactyloids is locked up pretty tightly thanks to their vertebrae fusing together, their scapulocoracoids being tightly braced between their sterna and backbones and a series of robust, mostly immobile ribs. How were their lungs or air sacs inflated within such a rigid skeleton? Because this configuration isn't a million miles from the torso skeletons of some birds, some authors (Claessens et al. 2009) have suggested that pterosaurs may have breathed in basically avian manner: muscles anchoring to small ribs set between the sternum and larger thoracic ribs move the sternum up and down, pumping air around the body in the process.

This has been accepted fairly widely for the last five years, but now Geist et al. (2014) have presented an alternative argument. They suggest that pterosaur sternal ribs are ill suited for anchoring such muscles because they are very slender - we might even call them fragile - and often entirely cartilaginous, the latter observation borne out by their poor representation in fossil record. Indeed, large portions of the pterosaur chest seem cartilaginous and rarely preserved - the bony sterna of many species (Dorygnathus and Scaphognathus spring to mind) are tiny, and cannot possibly have supported the flight musculature indicated by their powerful shoulders and forelimbs. There must have been large cartilage extensions to these in life. Moreover, in many respects pterosaur torso construction resembles those of crocodilians more than birds, such as the manner with which the thoracic ribs articulate with the vertebrae and the essentially vertical orientation of the ribs themselves. This configuration does not permit the rib rotation required to move the sternum in respiration, and actually adds further rigidity to the anterior pterosaur torso. A bird-like respiratory mechanic may be unlikely for pterosaurs then.

So how were pterosaurs breathing, then? Perhaps the only part of their bodies which wasn't locked up tight and permitted the expansion and contraction required for breathing was their bellies. Behind the sternum sits a suite mobile bones: the belly ribs (gastralia) and the prepubes, a pair of paddle-shaped bones articulated with the pelvis, along with a few 'floating' sternal ribs. Perhaps, like crocodiles, but unlike birds, pterosaurs used this region of their body to control the pressure in their lungs. Crocodiles use contraction of their abdominal muscles to move a large, body-spanning liver forward to compress their lungs, while relaxation of their abdominal wall then allows the liver to retract and the lungs to expand, bringing in their next breath. It seems this action accounts for about 65% of air moved in and out of their lungs, with the rest coming from costal - rib - movements. Given that it seems only pterosaur bellies were flexible enough to inflict substantial changes on body volume, it is not inconceivable to think they used a similar 'belly-pump' (or extracostal pump) as their principle means of controlling air flow into their lungs.

What does this mean for pterosaur lung structure overall? It's well known that pterosaur skeletons and bodies were pneumatised to the same extent, if not more, than avian dinosaurs, prompting suggestions that pterosaurs also had solid avian-like lungs and similar unidirectional flow-through pulmonary mechanics (Claessens et al. 2009). Do the observations of Geist et al. (2014) refute this? Well, not really, but they don't support them, either. As Geist et al. point out: we really don't know anything concrete about pterosaur lung structure, and it's actually pretty hard to tell anything about them from bones alone. A bird-like lung may have been present in pterosaurs and would certainly be consistent with extensive skeletal pneumaticity. However, we need to be careful about exclusively linking extensive pneumaticity with bird-like respiratory organs: flying fish, which of course have no lungs at all, also have pneumatised skeletons thanks to outgrowths of their swim bladders (Geist et al. 2014). Moreover, our uncertainty is not helped by a general lack of knowledge about reptile lungs. This year has seen several revelations about the lungs of extant reptiles being more complex, and sometimes more avian-like, than previously thought. We might need a better handle on reptile lung diversity, and the phylogenetic distribution of different lung structures within Sauropsida, before we start making inferences about the lungs of long extinct reptile lineages. In sum, while the avian-like pterosaur lung remains a viable hypothesis, it's not the only option on the table. We might be able to gain insights into how the body cavity of pterosaurs was manipulated to move air in and out, but their precise lung anatomy remains largely mysterious (Geist et al. 2014). There's a lot more we could say about this, but you'll have to track down the full paper for further details.

The cover image

Festivodactylus in situ.

There's a bit of a story behind the cover artwork for this paper too. It's hardly the stuff of novels but, given that 2014 has been another year in which palaeoart plagiarism and working practices have been a hot topic, it's nice to share a happier, positive story about a palaeoartwork for a change.

This cover has been a long time coming, with Nick asking me for potential cover art for the paper at the end of last year. I duly obliged by lending the flying Anhanguera from my book. Nothing much happened while the paper was crunched through the publication mill, until in November the cover art arrived. Looked like I was due for the December issue, which, as noted above, Anatomical Record always jazzes up with festive colours - green, red and white. This involved tweaking the colours of my original art to meet these, as well as some stretching and cropping to fit the AR cover format. Without going into details, I wasn't really happy with the results. Uh oh. Pessimist I am, I foresaw the worst. I stress that these expectations weren't because of previous experience of working with Nick or AR, but my experiences with other clients and agencies. Protesting about art use normally leads to Bad Things: unhappily forced compromises, loss of commissions, or having to fix 'problems' without pay. When writing back to Nick and AR with my concerns, I pretty much expected the whole cover project to fall apart. I pitched, without optimism, the idea of doing another image, for a fee, to replace the modified one. Despite linking to the 'State of the Palaeoart'article I helped pen this year to substantiate my request for payment, I was expecting the same old response: lack of money, thanks but no-thanks.

To my complete surprise, Nick, his colleagues and AR were on board with everything. The 'palaeoart situation' was new to them all, but I - we, the palaeoart community - had their sympathy. Within a day, AR had been able to put things on hold for a week while I drafted a new image to their specifications and size, Nick and his team rapidly found a generous payment for the work at short notice, and we all ended up with a product we were happy with.

I mention all this for two reasons. Firstly, Nick, his team and AR deserve accolade for being so refreshingly cool and respectful of palaeoartistry. Secondly, independent palaeoartistry can seem a most hopeless industry at times: we get ripped off by everyone from toy companies and movie makers to museums and publishers; our marketplace is mainly structured around exploitation of individuals, and sympathy or assistance from those in the position to change this can be hard to find. But, as this case shows, it's not all hopeless. Increasing awareness of the issues facing palaeoartistry does help rectify them, change can happen, and we have more supporters than we know. I'm optimistic that eventually we'll all have more stories like this one than the negative situations currently reported so frequently.

Friday, 5 December 2014

This week I've been mostly rendering an abelisaurid, the theropod group best known for short-faced, short-armed taxa such as Carnotaurus, and famous for occupying many predatory niches in southern continents while tryannosaurids occupied the north. This 'common knowledge' is only mostly true however: abelisaurids did spread to at least Europe in the uppermost Cretaceous, and represent the largest predatory dinosaurs on the continent at that time. We need not overstate that significance however: uppermost Cretaceous Europe was a topsy-turvy world where all theropods were rather small, and top-dog predator duties were likely filled by terrestrially-stalking, quarter-tonne azhdarchidpterosaurs. It seems even evolution likes a good Htichcockian-twist every now and then.

In southern continents, abelisaurids remained medium-to-large sized predators. It was one of these, the Campanian, Patagonian species Aucasaurus garridoi that Felix Bridel asked me to paint as one of my £100 palaeoart commissions. Aucasaurus is one of the best known abelsisaurids, the holotype skeleton presenting an almost complete osteology of this c. 5 m long animal, and is considered a 'derived' member of the abelisaur clan. Its anatomy is almost as unusual as that of its close relative Carnotaurus. Like other abelisaurids, the general bauplan of Aucasaurus was that of a small head, tiny arms and relatively gracile torso strapped to a robust, probably powerfully muscled hindlimbs and tail. The peculiar anatomy of derived abelisaurid proximal tail vertebrae created huge spaces for hindlimb muscle-anchorage and likely betrays tremendous sprinting power (Persons and Currie 2011). Perhaps this explains the strange upper-body anatomy of abelisaurids: their gracile bodies and reduced extremities may represent pressures to keep weight down and speeds high. Long term readers may reconcile some components of this functional complex with another group of dinosaurs, the ornithischian clade Dryosauridae, which also married a powerful set of legs with a svelte upper body.

Felix wanted to incorporate the notion that Aucasaurus and similar species were probably fast runners into his commission, but not in a conventional way. While recent research has hinted that abelisaurids were likely fast, they were likely not as agile as other taxa. Bulging abelisaurid hindlimb muscles may have created a lot of power, but they also kept the tail base rather immobile (Persons and Currie 2011). In tight turns, the tail probably fairly ineffective for adjusting gravity centres or resisting turning inertia. Felix wanted his Aucasaurus image to reflect this, showing a predator which had overcooked its pursuit of a more nimble prey item and lost balance altogether. I was more than happy to oblige: watching animals for any length of time reveals they are just as clumsy as we are, but we seldom see palaeoart reflecting this. Perhaps the only exception is predators being thwacked by the weapons of their prey, which I guess sort of reflects a clumsiness, but it's not quite the same level of slapstick as an animal slipping over.

My brief working time with Felix was a lot of fun: it was clear from our first few emails that we had a similar idea in mind and the image came together quickly. I posted my progress of the image up on Twitter and, as you can see below, the basics of the image weren't altered from the start. Efforts were made to show the Aucasaurus mid-slip: deep enough into the fall for some immediate reaction to show, but early enough that the limbs and dust still have to settle. To my mind, the animal's right leg flew out from beneath it when attempting a tight left-turn, leaving it to gaze off-canvas at whatever prey item it was pursuing.

That final Tweet did indeed feature the finished version (also seen at top), which is now being printed and packaged for delivery (click here if you would like your own copy). As usual, there's a lot more to say, but I'll have to end there. Before I go, a few nods are needed as goes sources for the picture: Scott Hartman's Aucasaurus skeletal was an important reference, as was the Coria et al. (2002) description of the Aucasaurus holotype. The notion that abelisaurid arms were used as display structures (termed 'pom pom arms' by @Blackmudpuppy) isn't new: All Yesterdays (Conway et al. 2012) explored that first.

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Who is this 'Mark Witton' chap?

Dr Mark Witton is a palaeontologist and palaeoartist, affiliated with the University of Portsmouth, UK. My technical research is focused on pterosaurs - Mesozoic flying reptiles - but my artwork has introduced me to a wide array of different fossil animals that are just as interesting. I work as a freelance author, consultant and artist: check out my work at MarkWitton.com, follow me on Twitter @MarkWitton, and browse my books here. Contact me at wittonprints[at]gmail.com. Due to volume of email I can't always reply to messages, but I do my best.